stereo cartridge on mono record

yes you are fine.

That will depend on the age of the mono record. Pre 60's mono maybe not good, as the needle profile if different. If 60's and later should be fine. The album cover could say ok for mono or stereo needle. Some record companies have two numbers like these CL or CM then numbers on the album cover for either mono or stereo, these should also be ok.

The follow link shows how to to wire a stereo cart for mono playback, you might want to take a look at it.

http://pagesperso-orange.fr/jlf/erepro2.htm

A stereo cartridge is sensitive to vertical modulation of the groove. A mono record has only horizontal groove modulation, and whatever vertical movement occurs (and this includes turntable rumble) is pure noise, and you don't want it. It is essential to remove sensitivity to vertical modulation by jumpering the two channels together, or using the MONO switch on your preamp (if it has one).

Improved results may also come from using a different (larger) stylus, and these are available for some stereo cartridges.

you are fine fine fine

When Stereo records were introduced in 1958, we all had collections of mono LP records and played them one and all with the new stereo cartridges, whose needles were some 70% of the diameter of the old mono LP needles. We were always cautioned, however, that damage would result if stereo records were played with the old mono needles. Soon even mono record players came with stereo cartridges. Todays computer designed precision cut diamonds are far, far, better than anything we had back then, and dig out much more information, while being much gentler on the record groves. Despite theory, I generally prefer the sound without engaging the mono button.

Samujohn...It isn't "theory". Vertical groove modulation in a mono recording is pure noise. You are saying that you like some noise mixed in with your music.

Try this...
Connect one channel of your cartridge inverted.
Jumper the two channels using a Y interconnect or a MONO switch. (Now you have a cartridge that is insensitive to horizontal groove modulation and sensitive to vertical).
Listen to your mono LP. What you hear is what is mixed with the music when you use a stereo cartridge without a Mono jumper.

Hi Eldartford,
you say:
>>> It is essential to remove sensitivity to vertical modulation by jumpering the two channels together, or using the MONO switch on your preamp (if it has one). <<<

Any idea at all, why when I use that 'summing' to mono on my ML326S (with phono) the level drops by at least 3dB? I seem to get rid of that "vertical noise" but also loose resolution (using a stereo MC).

as Samujohn says:
>>> Despite theory, I generally prefer the sound without engaging the mono button. <<< Could it be because of what I mention above?

I'm sure that the 'jumpering' must work as Eldartford
says, but brother ---- that's a pain, if you do not have a lot of MONO to listen to.
I have a suspicion that if the phasing L/R (due to stereo cart performance) is not at it's best, the summing will cause some signal cancellations and that would explain the drop in output level, correct me if I'm wrong.

Thanks,
Axel

samujohn is correct. this is a case where 'a problem is invented or exagerated to provide a grand solution'.

Jaybo...If you had ever listened to the L-R signal when a mono LP is played you would not call the problem "theoretical".

By the way, you might try listening to the same L-R signal from a stereo FM radio broadcast. The L-R signal is separately broadcast as a FM signal between 20 and 40 KHz, demodulated, and mixed with the L+R mono signal to get L and R. This scheme only works well with a very strong RF signal, which is why in weak signal areas FM mono works fine but stereo is lousy.